posix: Add cpu_hold() function to better emulate code delay

In native_posix and nrf52_bsim add the cpu_hold() function,
which can be used to emulate the time it takes for code
to execute.
It is very similar to arch_busy_wait(), but while
arch_busy_wait() returns when the requested time has passed,
cpu_hold() ensures that the time passes in the callers
context independently of how much time may pass in some
other context.

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>